1. Field of the Invention
The present invention relates to a stereoscopic three-dimensional (3D) camera. Such a camera may be used in consumer and professional photography, 3DTV, police identification, medical imaging, and scientific visualisation.
2. Description of the Related Art
Human beings have two eyes 1 and 2 as shown in FIG. 1 of the accompanying drawings. The eyes are disposed side by side, separated by a distance that varies from person to person but averages at about 65 mm. The eyes see a three dimensional world from two slightly different points of view as illustrated at 3 and 4. Objects such as 5 and 6 close to the eyes are seen from different angles and such objects appear shifted, relative to more distant objects such as 7, when the views from both eyes are compared as shown at 8. This shift is called xe2x80x98Parallaxxe2x80x99. This parallax is dependent on the distance from the eyes such that the further away the objects are, the smaller is the shift or parallax.
This is known as xe2x80x98Binocular Visionxe2x80x99 and enables a person to judge distance to an object and thus assess size when no other cue (such as motion, memory or perspective) exists to judge the distance. This ability to judge the distance is called xe2x80x98Stereopsisxe2x80x99, meaning xe2x80x98solid seeingxe2x80x99.
The discovery of stereopsis and photography were put together early. If a camera could mimic one eye and produce a 2D photograph, then it was assumed that two cameras, set up at a similar distance apart as the human eyes, could mimic the 3D stereopsis effect as well. Each photograph mimics the image taken from each eye with the corresponding parallax and shifts necessary to judge distance by stereopsis alone. The two photographs thus obtained would need to be displayed to an observer with one photograph to one eye and the other to the other.
A small system of lenses achieved this. Known as a Stereoscope, it was quite popular among upper and middle class families by the 1850""s. It was not long after the invention of moving pictures that moving 3D films were shown.
Many methods of display are known including the stereoscope, anaglyph (red/green glasses) and LCD switchable glasses, autostereoscopic methods (where no glasses at all are required), head tracking displays and virtual reality. However, any display can be only as good as the photographs or images that are presented to it.
FIG. 2 of the accompanying drawings illustrates the xe2x80x9creal worldxe2x80x9d situation where the eyes 1 and 2 look at objects 6 and 7 at different distances from the eyes. When the eyes are looking at the distant object 7, the xe2x80x9coptical axesxe2x80x9d of the eyes converge on the object 7 and the eyes focus on a focus plane 9 at the object. Conversely, when the eyes focus on a nearer object 6, the optical axes converge and the eyes focus at another focus plane 10.
FIG. 2 also illustrates what happens during 3D viewing of a display having a display screen 11. Irrespective of the apparent distance of the images 12 and 13 from the viewer, the eyes 1 and 2 must maintain the display screen 11 in focus. Images such as 12 which appear in front of the screen have negative parallax whereas images such as 13 which appear behind the screen have positive parallax. Although the eyes 1 and 2 must maintain the screen 11 in focus, eye convergence has to vary according to the perceived distance to the image. Thus, in a 3D photograph, the parallax defines the convergence in the usual way but the eyes must have the fixed distance to the photograph or screen remaining in focus.
It is well known that a certain amount of mismatch between accommodation and convergence is considered to be tolerated comfortably, allowing 3D photography to function within limits of depth either side of the focus distance. This results in limitations to the parallax on the photographs in the horizontal direction. In addition, significant vertical parallaxes can cause serious viewer fatigue.
This places significant tolerances on the cameras, their design, construction and use. The parallax produced on the photographs depends on many variables, e.g. the separation of the cameras, zoom and field of view, convergence, display method, etc. They must all be controlled so as to keep both horizontal and vertical parallaxes on the two photographs within comfortable limits. In known systems, this requires specialist knowledge and experience on the part of the cameraman.
Many designs for cameras have been made. The three most common are a single camera moved on a slide, a single camera with extra mirrors and optics, and a two camera design. The following references provide representative prior art in this field.
xe2x80x9cFoundations of the stereoscopic cinemaxe2x80x9d Lipton. Van Nostrand, 1982 ISBN 0-442-24724-9 pages 114 to 115 described one form of the camera equation which is described hereinafter.
GB 2 250 604 refers to an adapter that can be attached to any camera. This adapter contains two mirrors and a zoom lens. The convergence of the mirrors and zoom compensation are controlled by computer to obtain two homologous images with maximal overlap.
WO 96/15631 refers to a method called xe2x80x9cDisparity Shiftingxe2x80x9d where two homologous images are superimposed as anaglyph with an offset in the x and y directions. This artificially changes the parallax on the 3D image to compensate for zoom, convergence, etc. changing the parallax.
GB 2 168 565 refers to a two-camera apparatus, adjustable according to separation, zoom and convergence. The zoom is optical and is effected on both cameras equally by a mechanism presented in the patent. The three variables can be controlled by microcomputer according to input data. No indication of how the relationship between zoom and interaxial separation is obtained for comfortable images is given. Further, the apparatus is limited to cameras whose optical axes converge so that this document teaches away from the use of parallel optical axes. A disadvantage with converging cameras is that they produce vertical parallax in the captured images (ie. keystone distortion).
JP 9-215012 discloses a 2D/3D video camera incorporating 2D and 3D video images. The images are displayed on an LCD panel on the camera are viewed using a separate stereoscope. The 3D images are captured by two fixed camera heads.
U.S. Pat. No. 4,768,049 refers to a method of positioning a single camera in two positions accurately using a slide bar, referring directly to stereo photography. Such a camera cannot capture accurate still images of moving scenes.
U.S. Pat. No. 5,063,441 discloses a stereoscopic video camera with parallel optical axes and camera image sensor elements which are adjustable so that the sensors are exposed to different portions of the images from respective lenses. However, the separation between zoom lenses of the camera is fixed.
U.S. Pat. No. 4,418,993 discloses an arrangement for controlling convergence and separation so as to maintain an object at the image centre when zoom is effected in order to reduce the parallax of the object to comfortable limits. However, simultaneously control of three parameters is required and, because of the convergence of the optical axes, acceptable results cannot be obtained with distant backgrounds. Converging the cameras also produces unacceptable results due to keystoning. Also, such a system is expensive and complex and is not easily controlled by an unskilled user.
According to the invention, there is provided a three dimensional camera comprising: at least one imaging and detecting device which is moveable in a direction substantially perpendicular to the optical axis thereof; defining means for defining a depth range of a scene whose image is to be captured at first and second positions; and deriving and limiting means for deriving as a function of the depth range a separation between said first and second positions and for limiting the separation such that the parallax between images captured at the first and second positions is less than a predetermined maximum parallax, wherein the optical axes of the or each device in said first and second positions are parallel.
The at least one device may comprise at least two imaging and detecting devices whose optical axes are substantially parallel to each other.
The camera may comprise a visual indicator for indicating whether a stereoscopic image to be captured is within predetermined parallax limits. The indicator may comprise a viewfinder or an indicator light.
The predetermined maximum parallax may be a function of a predetermined display geometry on which captured images are intended to be displayed. The predetermined display geometry may comprise display screen size, intended viewing distance and eye separation.
The defining means may be arranged to define at least one fixed depth range limit. The at least one fixed depth range limit may comprise infinity. The at least one fixed depth range limit may comprise a camera minimum focus distance.
The defining means may comprise first selecting means for manually selecting the depth range. The first selecting means may be arranged to provide manual entry of the depth range or to select from a plurality of predetermined depth ranges.
The defining means may be arranged to measure at least one depth range limit. The defining means may comprise an auto-focus sensor. The at least one depth range limit may comprise a near depth point. The defining means may comprise second selecting means for manually selecting from a plurality of fixed far depth points.
The or each device may have a variable field of view. The camera may comprise third selecting means for manually selecting one of the separation and the field of view and fourth selecting means for automatically selecting the other of the separation and the field of view as a function of the one of the separation and the field of view.
The function may be a monotonically increasing function.
The function may also be a function of the maximum and minimum distances from the camera of objects whose images are to be captured.
The function may also be a function of the geometry of a display for displaying images captured by the camera. The camera may comprise means for supplying to the fourth selecting means data representing the display size and viewing distance.
The devices may be substantially identical to each other.
The detectors of the devices may be disposed in a common plane substantially perpendicular to the optical axes.
The detector of at least one of the devices may be offset laterally with respect to the optical axis of the device.
The field of view of each device may be selectable by selecting a portion of the detector.
The field of view of each device may be selected by adjusting the focal length of an imaging optical system of the device. The devices may comprise zoom lenses whose optical nodes are disposed in at least one common plane perpendicular to the optical axes.
One of the devices may be fixed.
The camera may comprise more than two devices with the spacings between adjacent pairs of the devices being maintained substantially equal to each other.
The detector of each device may be rotatable through substantially 90xc2x0 about the optical axis of the device.
The detector of each device may comprise a charge-coupled device.
It is thus possible to provide a 3D camera which is capable of capturing images over a wide range of distances from the camera. By limiting the maximum parallax which is captured, subsequently displayed captured images are viewed comfortably by a viewer. Such a camera may provide hard copies, for example in the form of photographs, or may supply images which are stored electronically, for example in semiconductor memories, floppy disks and CD ROMs. The camera may also be in the form of a video camera for capturing moving 3D images, for example for display on a suitable video rate 3D display.
A user need not have specialist knowledge in order to obtain 3D images which are suitable for display while providing good-quality 3D appearance. For example, in one form, the user manually changes the separation of the device and the field of view or zoom setting is automatically calculated and applied using, for example, pre-set or manually input values of display geometry, human factors and/or distance ranges. For cameras including autofocus systems, such systems may provide distance data directly. In this case where the user manually selects the device separation, the zoom function maintains the same parallax conditions for viewing. It is thus possible to provide a compact 3D camera which is easy to handle.
Extra-wide base line 3D photographs or other captured images are possible if one device is detachable or extendable, for example on a slide. Also, objects with maximum depths of less than infinity can be imaged into a more useful depth on a display.
Such a camera may also be used in a two dimensional (2D) mode, for example to allow 2D digital photographs to be easily obtained. Furthermore, one or more of the devices may be xe2x80x9ctoed-outxe2x80x9d so as to allow an extra wide 2D panoramic picture to be obtained, for example when processed by means of seam matching software.